Not seen as much of M'Vila as I have of Martinez. Only really seen him playing for France, he's looked very good, where he anchors the midfield. He's a very good passer of the ball but seems more of a holding ball winner although I've heard he's played a more expansive role for Rennes this season. Martinez offers more goals and assists, seems to get around the pitch more. Despite playing most of this season at centre back he's scored 4 goals to M'Vila's 1.
Depends on what you think we need. If it's another disciplined holding player to sit alongside Carrick in a 4-2-3-1 system then maybe M'Vila is the better player. If we're carrying on with a 4-4-2 then Martinez would be the better signing.
Would be very happy with either. They'd both improve our midfield a lot.